Degradation of algal lipids in microcosm sediments with different mixing regimes

A series of microcosm experiments was conducted to examine the influence of different mixing processes on algal lipid degradation in surficial sediments. The experiments simulated 3 different mixing regimes: bioturbated, episodically physically mixed, and no mixing. Uniformly 13C- and 15N-labeled algal cells were deposited on the sediment-water interface to simulate natural deposition of bloom-produced organic matter. Degradation rate constants of individual algal lipids, effects of macrofuana species and physical mixing frequency on these rates, and the response of the microbial community were examined during one-month incubations. Our results showed that different mixing processes established different biogeochemical degradation regimes, characterized by variable redox conditions. Algal lipids degraded differently in different mixing regimes. Slower degradation was observed under episodic physical mixing because substrates were mixed deep into the sediment where anaerobic conditions prevailed. The degradation rates of algal lipids subjected to oscillating redox conditions during bioturbation were similar to those in unmixed cores where aerobic conditions dominate at the surface. Our laboratory results support the OET (oxygen exposure time) model based on field observations: longer residence of organic matter in the oxic zone leads to low preservation. Biological and physical mixing processes in surface sediments affect degradation and preservation of organic matter through different controls on redox conditions.